Optical communication systems, systems using optical signals to convey information across an optical waveguiding medium, are well known in communication systems. Such optical systems include, but are not limited to, telecommunications systems, cable television systems, and local area networks.
As the need for broadband communication services increased, it became necessary to increase the capacity of known waveguiding media. One solution is wavelength division multiplexing (WDM), which utilizes plural optical signal channels, each channel being assigned a particular channel wavelength. Signal channels are generated, multiplexed, transmitted over a single waveguide, and demultiplexed to individually route each channel wavelength to a designated receiver.
Conventional broadband systems can be configured in one of two ways. A general structure is set forth in FIG. 1. As can be seen, a typical system includes a plurality of narrow band tunable lasers, such as DBR lasers, located at the Central Office 1 where data, TV, voice signals, etc. are modulated onto a preassigned wavelength. The combined wavelengths are fed onto the feeder fiber 2. At the remote terminal, each wavelength is separated into the distribution fiber associated with that wavelength by a WDM demultiplexer element 3 and provided to the appropriate user. Conventional dense WDM multiplexers/demultiplexers support up to 20-30 wavelengths.
According to one configuration, each user is preassigned a particular wavelength, and all data addressed to that user is fed over that wavelength. In this case, a wavelength filter is provided associated with each receiver, so that each user receives only the information modulated onto his wavelength. According to another configuration, different services are assigned unique wavelengths. If a WDM element is provided at the remote node, no filtering element is required at the optical network unit (ONU). On the other hand, if a passive splitter is provided at the remote node, all the wavelengths transmitted to the users and defined service or services could be separated by tunable or passive filters at the ONU. Alternatively, different service groups could be provided according to their wavelengths. In this case, different services in each group could be combined and modulated onto the one wavelength.
However, known WDM systems typically include multiplexer and demultiplexer units which permit only a fixed number of optical channels to be used in the optical system. In order to expand such systems to include additional optical channels or wavelengths so as to add more users or services, additional multiplexers and corresponding demultiplexers must be added to the svstem, which is very expensive.
One solution to this problem was proposed by Alexander et al., in U.S. Pat. No. 5,557,439. This patent proposes an expandable wavelength division multiplexed optical communications system including an optical multiplexer module with N+x inputs, where N is the number of source lasers and corresponding optical signal channels, and x is the number of supplemental input ports to which no source laser is coupled. As additional channels are required, a new source laser with a corresponding optical signal channel not yet in use can be coupled to one of the supplemental input ports. However, such a system, which includes more channels than required, results in wasted channels and higher cost at start up, when fewer channels are needed.
Access networks differ from other broadband systems in that the bandwidth requirements for data transmission from customer to central office (upstream) may be different or much less than that for downstream transmission (from central office to customer). Different services can be provided with their assigned wavelengths. In this case, the number of required wavelength channels for downstream can be less than for upstream, where each customer is assigned his own unique wavelength. Thus, it is very important for access systems to have a flexible way to add additional wavelengths in response to demand.
At present, bulk or integrated optical grating-based demultiplexers are used in WDM systems in order to perform channel demultiplexing. These components can support at least 20-32 wavelengths. However, the component is very costly, the price per wavelength channel being in the range of $700-$1500. For transmission networks based on WDM technology, this factor is less important, since a main WDM advantage is to provide more capacity through each fiber. In access systems, on the other hand, the advantage of the WDM is the aggregation of services, service providers, different protocols on a single network, as well as providing additional capacity. It is clear that it is very important to add more wavelengths only according to demand, in order to decrease investment and to achieve a linear network roll-out, in which a network evolves gradually by adding the same unit each time when adding more services (wavelengths), service providers, etc.
In order to add information or drop signals, add and drop filters are known. In modern WDM systems, these add and drop filters are optically based, and signals on different wavelengths can be added or dropped. One example of an optical add and drop filter includes a pair of fibers connected at two points along their length, and a pair of identical Bragg gratings resonant at an identical wavelength, one of the Bragg gratings being disposed in each of the fibers between the two connection points. These systems are used today to permit communication between a plurality of remote nodes and a central office, without one interfering with the other. In this system, each remote node has its own assigned wavelengths for receiving and transmitting, which can be the same or can be different. Data for all remote nodes is transmitted over the fiber, each on its own wavelength. When data at the wavelength of a particular location is received, its first Bragg grating reflects that wavelength (it is dropped) and the output is received in the remote node. When the remote node wishes to send a communication, its data is overlaid on the same wavelength, and added to the system through the second Bragg grating.
There are also known systems for transmitting data over a single fiber in two directions. In order to permit such transmission without interference between the two signals, a loop is formed in the fiber and a so-called fiberoptic circulator is inserted between two points along the fiber. Signals traveling in one direction pass directly through the circulator, while signals traveling in the opposite direction are diverted around the loop.
Accordingly, there is a long felt need for a relatively inexpensive expandable multiplexer and it would be very desirable to have such a multiplexer which can be expanded several times in a modular fashion, particularly for use in access networks,